1. Field of the Technology
The present technology relates to a nonaqueous electrolyte and a lithium ion rechargeable battery.
2. Description of the Related Art
Due to high energy density thereof, lithium ion rechargeable batteries have been widely used as power sources for portable electronic devices, such as video cameras, mobile phones, notebook PCs and mini-discs, for which miniaturization and weight reduction are progressing.
Further, since the energy efficiency thereof (power efficiency) during charging and discharging is high in comparison with lead storage batteries or nickel-metal hydride batteries, lithium ion rechargeable batteries may also be used in electric vehicles or for power storage purposes and development regarding medium and large size batteries is actively being pursued.
As the lithium ion rechargeable batteries, ones including an anode formed of a crystalline carbon material such as graphite having excellent lithium occlusion and emission, a cathode formed of a complex oxide of lithium and transition metals, and a nonaqueous electrolyte, are most commonly used. As the nonaqueous electrolyte, for example, ones in which lithium salts such as LiBF4, LiPF6, LiN(SO2CF3)2 or the like are added to a mixed solvent (nonaqueous solvent) of highly dielectric cyclic carbonates, such as propylene carbonate and ethylene carbonate, and low viscosity chained carbonates, such as diethyl carbonate, methyl ethyl carbonate, and dimethyl carbonate, are in general use. However, cyclic carbonates, in particular, propylene carbonate, are reduced and decomposed at the electrically active anode surface, and the occlusion and emission of the lithium regarding the graphite may be disturbed. As a result, the internal resistance of the battery is increased, voltage fluctuations become large, the charging and discharging characteristics of the battery are deteriorated, and the battery life is reduced. Further, electrolytes including ethylene carbonate harden at low temperatures, and the ion conductivity thereof is deteriorated. The deterioration of the ion conductivity of the electrolyte causes an increase in the internal resistance of the battery as well as deterioration in the charging and discharging characteristics.
In consideration of the above-described problem, for example, vinylene carbonate, ethylene sulfide or the like is added to the nonaqueous electrolyte including the highly dielectric cyclic carbonates and the low viscosity chained carbonates. These additives prevent the decomposition of the cyclic carbonates and improve the charging and discharging characteristics of the battery by forming a protective film having ion conductivity on the anode surface. Further, there has been proposed a nonaqueous electrolyte containing pyrocarbonate having an alkyl group as the nonaqueous solvent (for example, refer to Japanese Unexamined Patent Publication JP-A 6-333594 (1994)).
However, at present, since there is a demand to further extend the operating time per charging of the lithium ion rechargeable battery, there is a demand for a lithium ion rechargeable battery for which deterioration of the nonaqueous electrolyte due to the charging and discharging cycle, increases in the internal resistance, and voltage fluctuations are small, and which is capable of stably exhibiting high level charging and discharging characteristics across a wide temperature range from a low temperature (for example, approximately −10° C. to 20° C.) to a high temperature (for example, approximately 60° C.)